Not ranty at all, don't worry. Schools are a perennial discussion/griping point for Londoners so if anything you are fitting right in with the English way of life by joining in 
My two-pennies worth - essentially where you have a densely populated city like London and when, inevitably, some schools are more popular than others, you have to have some way of deciding who goes where - whether or not you agree that faith/grammar schools should be allowed at all (or allowed to use faith/exam performance as an entry criteria if they are), the fact that all schools use proximity/distance as a criteria doesn't work well here, although it may be fine in smaller towns and rural areas. In London, tiny catchment zones mean you create a sort of two tier system by default, as richer parents are able to pay more to move house/live in more expensive catchments - and really it's not necessary/sensible to use distance in this way as schools tend not to be very far from one another in relative terms. If you truly believe in comprehensive schools I think a lottery system would be fairer so anyone within the borough/within a certain (fairly wide) distance of the school would have an equal chance of attending, but obviously this would mean some kids missed out on going to the 'better'/more middle class schools and would have to go to the 'rougher'/poorer schools further away which obviously isn't desirable to the middle classes (who have a lot of voting power etc!). Implementing this would affect house prices as well as affecting children's education so you can imagine the uproar, but eventually it would mean less gaming of the system, evening out of house prices so you didn't get absurd situations where identical houses half a mile apart cost vastly different amounts, schools would be more truly comprehensive as there would be more of a spread of different backgrounds and abilities in each school, and parents who want to buy their way into a better education would be forced to do so honestly by paying for private school. It would make measuring performance easier too, as at the moment a lot of the schools which get very good exam results are massively helped by the fact their intake is predominantly the children of middle class pushy parents whose children are already far likelier to do well regardless of how well the school educates (there are progress/attainment/balanced measures than exam results as well but can be harder to interpret). For now however, the system is as it is...
For what it's worth, I think you have plenty of choices here and none of them are bad - you can buy the biggest/best house in your favourite area and worry about schools when the time comes (secondary is still a good few years away for your DS and schools and catchment areas can change a lot in the course of a few years), you can compromise and buy somewhere smaller in the catchment zone of an outstanding comp, you can tutor for grammar schools (this doesn't have to be the 5 hours a night enforced slavery, 'your-life-depends-on-this' nightmare you see some parents putting their kids through, for a bright kid a little less intensive preparation can be plenty and you don't need to pressurise them - they don't need to get the highest mark in the country to get in after all, and they don't need to see how much you want them to go to a particular school) or you can buy somewhere modest with your capital and save everything you would have spent on the mortgage towards private fees (probably this would mean both a flat rather than a house and zones 3-4 rather than 2-3 but still a nice standard of living overall).
I'm sure your DS will do very well wherever he goes, the biggest determinant of a child's academic success is thought to be how involved and educated their parent(s) is, he will take his expectations etc from you. There is no comprehensive school in London, even the very worst/roughest ones which will rule him out from going to a RG university and getting a professional job. You see stories in the local papers every single year of students from very average/'rough' comps who have worked hard, got top grades and go to Oxbridge - yes these students may be the exception rather than the 'norm' as at a top public school or grammar but it's clearly possible and those kids will have benefited from a whole different system of education going through a comp rather than a selective school, it develops different qualities IMO. I guess it depends a bit on your DS' personality though and there shouldn't be shame attached to trying to get the best education possible for him, is he the type that likes being the big fish in the small pond or is he happy to be 'middling' and needs pushing to realise his potential? What type of atmosphere would suit him best socially? What are his extra curricular interests (a lot of what private schools offer over and above state education are extra curricular e.g. better music/drama/sports facilities, but arguably you can recreate a lot of this through paying for tutors/clubs etc outside school)? Not that you have to have all the answers now but things to think about...